What is coevolution?
- reciprocal evolutionary interactions between pops belonging to two entities- leading to the change in allele frequencies in both pops (host pathogen)
Types of coevolution
- Mutualistic= evolved mutually beneficial relationships
- Antagonistic= adaptations in 1 species matched by counter adaptations in the other- aim to overcome
- Red queen dynamic= running to stay in same place- pathogen and host constantly evolving to maintain advantage
Would a pathogen potentially evolve faster or slower than a host?
faster, due to short generation times and faster mutation rates
Which type of coevolutionary dynamics are best suited to simple versus
complex pathogens?
simple pathogens more likely to experience selective sweeps due to higher mutation rate
What are the requirements for co-evolution to occur?
- genetic variation exists for relevant host and pathogen traits
- traits of both interacting species are detrimental to each others fitness
- outcome of pathogen interaction depends on combination of genes involved
What are the two main GENETIC MODELS
of host-pathogen interactions?
- Gene for gene model
2. Matching alleles
How do the genetic models RELATE to
coevolutionary dynamics?
- Gene for gene model- The model predicts a universally virulent parasite, associated with arms race dymamics
- Matching alleles- Leads to Fluctuating Selection Dynamics, resulting continued genetic polymorphism
How can we study coevolution
EXPERIMENTALLY?
time shift experiment
Why are the predictions of a TIME-SHIFT
experiment different for each type of
coevolution?
If coevolutionary changes were driven by selective sweeps;
- pathogens from past less infective, from future more infective
- larger time shift between the parasite and its host allows the parasite to have accumulated a larger number of adaptive mutations
If coevolution were dependent on fluctuating selection
- on average there should be no net increases in pathogen infectivity, or decreases in host resistance, over time
Explain the gene for gene model of host-pathogen interaction
-The parasite has avirulence (AB) and virulence genes (ab)- The host has susceptibility (ab) and resistance alleles (AB)
- Resistance occurs when at least one parasite ‘avirulence’ gene is recognised by a host ‘resistance’ gene
The model predicts a universally virulent parasite
- associated with arms race dymamics
Explain the Matching alleles model for host-pathogen interaction
- A parasite must exactly match a host’s genotype to elude the host’s immune system and successfully infect the host
- All parasites genotypes infect an equal number of host genotypes
- Leads to Fluctuating Selection Dynamics, resulting continued genetic polymorphism
What is the arms race dynamic (selective sweeps)?
A Coevolutionary dynamic
- Selective sweeps of novel host resistance and pathogen infectivity alleles
- Repeated fixation of new alleles for parasite infectivity or host resistance
- relies on mutation to introduce new alleles- occur very rare
What is fluctuating selection dynamics?
A coevolutionary dynamic
- Host and pathogen allele frequencies oscillate over time because of negative frequency dependent selection
- Rare alleles in host/pathogen have advantage
- Genetic variants are maintained in the population of a long period
